Friday, 22 June, 2012

Graphene research - Trapping light in a carbon net

Back-to-back publications in Nature by CeNS members Fritz Keilmann and Rainer Hillenbrand

Graphene, an ordered monolayer of carbon, is the thinnest substance known, and yet has extraordinary mechanical strength. A new study shows that its two-dimensional network of atoms can even trap light.

The high mobility of electrons in graphene arises from the fact that they are confined to the hexagonal lattice. An international team led by the American physicist Dimitri Basov has now shown that photons too can be trapped within the lattice and move freely along it. “It is even possible to control the light waves within the lattice,” says physicist Dr. Fritz Keilmann, member of the Center for Nanoscience (CeNS) and affiliated with both the Max Planck Institute for Quantum Optics (MPQ) and LMU Munich, who contributed significantly to the new work.

Theoretical calculations had previously suggested that photons, specifically long-wavelength infrared photons, could indeed be propagated along the graphene lattice at greatly reduced velocities. The slowing effect was attributed to the formation of plasmons, a type of hybrid particle produced by coupling of the photons to electronic excitations in graphene. It has not been possible to study the predicted plasmons experimentally, because the momentum of the infrared photons was far too low to excite them.

This problem has now been solved with the aid of a tiny metal pin with a nanometer-sized tip. Acting like a lightning rod, it concentrates incident light into a very small volume. This raises the momentum of infrared photons by up to 60-fold, giving them the extra momentum they need to launch plasmonic waves along the graphene layer. For this purpose, the researchers made use of CeNS spin-off neaspec’s infrared scanning near-field microscope NeaSNOM to image the edge zone of a graphene sample. Reflection of the plasmons at this edge produces an interference pattern, which encodes information that can be used to confirm formation of the plasmons. It also allows one to deduce many of their properties, such as the magnitude of the reflection at the graphene edge and the change in the velocity by external electric bias, which is of particular interest for future applications. “The long-sought ability to control light signals by electrical means has thus been realized,” says Keilmann.

An independent study, co-lead by CeNS member Rainer Hillenbrand (CIC nanoGUNE, San Sebastian), Frank Koppens (ICFO, Barcelona) and Javier García de Abajo (IQFR-CSIC, Madrid) comes to a similar conclusion. Their findings are based on the use of a graphene nanoribbons formed by deposition from the gas phase rather than the exfoliated film used in the LMU work, also applying Neaspec's NeaSNOM technology. Rainer Hillenbrand comments: "Seeing is believing! Our near-field optical images definitely proof the existence of propagating and localized graphene plasmons and allow for a direct measurement of their dramatically reduced wavelength." The two papers have now appeared back-to-back in the top-tier journal Nature, underlining the significance of these findings for the field of nanoelectronics.